The present invention relates to a device, an image processing device and a method for optical imaging which may, for example, be used in miniaturized camera systems for portable terminal devices.
The use of miniaturized camera systems for portable terminal devices (mobile telephone, PDA, laptop, etc.), apart from the miniaturization of electronic and optoelectronic devices, also requires the miniaturization of the imaging objectives or lenses. Preconditions for this are short lengths of the objective and a small number of optical components (in particular lens elements). The increasing reduction of the image sensor diagonal, which is supported by the development of semiconductor patterning technology (smaller photodiodes equals greater number of pixels on the same image area) and by the reduction of the sensor manufacturing costs, requires, however, that, in spite of making the construction of the optics more simple, a high resolution capacity and a high light strength of the optics have to be achieved. The existing optics design solutions are characterized by few, but complex (usually aspherical) lens forms which exhaust the possibilities of current manufacturing technologies. By unsatisfactory measurement methods for quality control of such complex areas and the highly precise lateral and axial mounting accuracies which are needed to take up the optical components of such a miniaturized camera lens or objective are restricted further when implementing the same. Existing solutions for miniaturized camera modules either do not meet the requirements of specifications or the expectations of integrators and users regarding costs.
A well established manufacturing method of small camera optics is the generation of single lenses and mounts by plastic injection molding in ultra precision processed mold inserts. Usually, here the lenses may be manufactured together with their mounts in a two-component injection molding. The individual components are subsequently mounted in a plugin mount and fixed by means of a positive connection (wringing in contact, adhering). This method may, however, not be applied for the manufacturing of miniaturized objectives with a building size of smaller than 5×5×5 mm3 in a sufficient adjustment accuracy. Further problems result for the supply and the mounting and connecting technology of such small components. In detail, problems regarding the handling of the components result due to electrostatic forces (small weight and dimensions of the devices) and the danger of contaminating and scratching the sensitive optical surfaces. For these reasons, more than 80% of production costs are due to assembly processes. There are advanced approaches regarding the handling of smaller optics in hybrid mounting technology (sensorically supported mechanical and electrostatic as well as pneumatic micro grippers), however the same increase the cost for large-scale manufacturing substantially (e.g. camera optics for mobile telephones). Further, by the hybrid manufacturing technology for higher resolution formats an active positioning, e.g. piezo actuator of the plastic optics, is required to balance the tolerances of mounting the objective on the optoelectronic image converter (image sensor). This leads to a further increase in the unit price.
An alternative method for objects in the size range smaller than 5×5×5 mm3 is the manufacturing of optics on wafer level (WLO wafer level optics). Here, a tool bit for the respective single lenses is used which was generated by ultra precision processing (e.g. diamond cutting), for a repeated UV replication (step and repeat process) of the individual component on a substrate wafer (wafer level optics modules). Alternatively, a complete tool wafer with the same individual components may be generated by means of ultra precision processing and be subsequently replicated in one single UV replication step on wafer level. Many lenses of the same type and also spacers and apertures may be manufactured in parallel in this way. In subsequent steps, the individual wafer plates may be axially bonded to each other to obtain a wafer stack with a plurality of objectives. This is a parallelized manufacturing technology using processes and systems of microelectronics manufacturing. The main disadvantages of using these manufacturing methods of micro-optics for miniaturized lenses which are, however, large as compared to conventional micro lenses, are the high costs of manufacturing suitable reproduction tools and the limited accuracy, for example due to the shrinkage of material, of achievable surface profiles in UV replication of micro lenses of high angular points (higher than 100 μm). Further, problems regarding reproducibility and quality testing remain, in particular characterizing the complex lens form of this size. The modules may hitherto only be tested in connection with all other optical components using an imaging method which strongly reduces the yield depending on the number of components and manufacturing steps.
Further, arrangements of a flat optical imaging sensor exist which represents the technical implementation of the apposition compound eye of insects. In this extremely compact, multi-channel imaging system, a photodetector (pixel) is associated with each micro lens.
In the following, a photodetector is partially also referred to as an image detector or also as a photodiode.
Due to the offset of the photodetector to the respective micro lens, despite the small size a very large visual field may be spanned. Due to the use of one photodetector per channel, there is, however, the need for a large area of the photodetector field (CMOS or CCD image sensor) to achieve a moderate image resolution capability. This considerably increases the manufacturing costs of a correspondingly miniaturized imaging sensor.
The documents DE 10 2004 003 013.8 and PCT PAT. APPL. WO 2005/069607 describe a multi-channel imaging system on the basis of an artificial compound eye, whereby here an image detector is allocated to each channel or a few image detectors with different functions are allocated to each channel. Each channel thus captures only a narrowly limited area of the object field.
The documents US 005696371 A and EP 0840502A2 describe a further multi-channel imaging system on the basis of artificial compound eyes. A compact digital camera with multi-channel refractive/diffractive imaging optics and a segmented visual field is described. The system consists of a field arrangement of lenses which are implemented as decentralized lens segments in whose focal length a photosensitive image sensor field is located. Axially ahead of the lens field two aperture fields with sloping side walls and a period greater with respect to the lens field are used to indicate the size of the visual field. For suppressing optical crosstalk, vertical walls of light-absorbing material are proposed between neighboring optical channels.
The document J. Tanida, T. Kumagai, K. Yamada and S. Miyatake, “Thin observation module by bound optics (Tombo) concept and experimental verification” Appl. Opt. 40, pages 1806-1813, April 2001, shows a further multi-channel arrangement for optical imaging. From this document, a multi-channel arrangement is known wherein the micro images of the optoelectronic image sensor are located centrally axially below the respectively associated micro lens and neighboring channels with vertical opaque walls are separated from each other. Using this arrangement, however, only a small object field may be detected. For small object distances (about less than 2 m), due to the occurring offset of perspectives (parallax) of neighboring channels regarding the same object point, a sub-pixel shift of the micro image imaging with respect to the photodiode group, channel by channel, of the image sensor, may be obtained which calculates a high-resolution overall image from the plurality of low-resolution micro images by means of a super resolution algorithm. This arrangement may, according to principle, only be used for small object distances and small object field sizes. Further, this method has an increased image readout and processing time, as the super resolution algorithm (known from image processing) has a high complexity.
An alternative technical possibility of circumvention is the use of very small photodiodes (pixels) for the optoelectronic image sensor in connection with one-channel optics. The consequently small image diagonal, with a short length of the optics, leads to small object field angles at the edge (small off-axis aberrations and low vignetting) and consequently also to the disadvantage of detecting only a small object field. For this miniaturized one-channel optics, a relatively small lens with a very small length and sufficient image quality may be used, so that also here the inventive (lithographic) technologies may be avoided on wafer level. However, small pixels have the disadvantage of a small photosensitive area and thus, with the same f-stop of the optics, of a lower sensitivity of the overall arrangement.
In summary it may be noted that there is no advantageous standard construction of an imaging device which unites a high image quality with a small height and may at the same time be manufactured cost-effectively by known micro-optical manufacturing technologies.